Fuel cells generate electric energy by an electrochemical within the cell reaction between a fuel (hydrogen source) and an oxidant (oxygen). Specifically, the chemical energy of the fuel is directly converted into electric energy. Examples of fuel sources which can be used include pure hydrogen and compounds containing hydrogen element, such as petroleum, natural gases (such as methane) and methanol.
Fuel cells have the following advantageous characteristics. A fuel cell itself employs no mechanical parts and, therefore, it generates little noise. Further, in principle, a fuel cell can semipermanently continue to generate electricity if fuel and oxidant are continuously supplied from outside of the cell.
Electrolytes can be classified into liquid electrolytes and solid electrolytes. A fuel cell which employs a polymer electrolyte membrane as an electrolyte is called a “solid polymer fuel cell”.
Solid polymer fuel cells are especially able to operate at low temperatures, as compared with other fuel cells. Accordingly, solid polymer fuel cells are expected as an alternative power source for automobiles and the like, as well as household cogeneration systems and portable electric power generators.
A solid polymer fuel cell at least comprises a membrane electrode assembly (hereinafter sometimes abbreviated to “MEA”) comprised of an electrode catalyst layer which is joined to both sides of a polymer electrolyte membrane. The term “polymer electrolyte membrane” mentioned here is a material which has strongly acidic groups, such as a sulfonic acid group or a carboxylic acid group, in a polymer chain thereof and allows selective permeation of protons. Examples of such a polymer electrolyte membrane which can be preferably used include perfluorinated proton exchange membranes, such as Nafion™ (manufactured by E.I. duPont de Nemours & Company Inc., U.S.A) having high chemical stability.
Examples of an electrode catalyst layer which can be preferably used include a thin sheet composed of a composite particle having an electrode catalyst particle supported on a carbon particle as illustrated in Non-Patent Document 1 and a catalyst composition consisting of a perfluorocarbon sulfonic acid resin as a proton conductive polymer (hereinafter referred to as “conventional electrode catalyst layer”). Further, if necessary, structures are also used in which the MEA is sandwiched between a pair of gas diffusion layers. In such a case, the laminated body consisting of the electrode catalyst layers and gas diffusion layers is referred to as a “gas diffusion electrode”.
For the operation of a fuel cell, a fuel (e.g., hydrogen) and an oxidant (e.g., oxygen or air) are, respectively, supplied to the anode side and cathode side gas diffusion electrodes, and the two electrodes are connected to each other through an external circuit. Specifically, when hydrogen is used as a fuel, hydrogen is oxidized on the anode catalyst to thereby generate protons, and the generated protons pass through a proton conductive polymer in the anode catalyst layer. Then, the protons travel in the polymer electrolyte membrane, pass through a proton conductive polymer in the cathode catalyst layer, and finally reach on the cathode catalyst. On the other hand, electrons which were generated simultaneously with the generation of protons from the oxidation of hydrogen flow through an external circuit and arrive at the cathode side gas diffusion electrode. On the cathode catalyst, the electrons react with the above protons and the oxygen in the oxidant to generate water, whereby electric energy can be obtained at this stage.
Such a solid polymer fuel cell is usually operated at around 80° C. in order to obtain high output characteristics. However, when used in an automobile, considering operation of the automobile in summer, it is desired that the fuel cell is able to operate under high temperature and low humidity conditions (operating temperature around 100° C. and a humidification of 60° C. (comparable to a relative humidity (RH) of 20%)). However, when a fuel cell employing a conventional perfluorocarbon sulfonic acid resin membrane as the polymer electrolyte membrane and a membrane electrode assembly consisting of a conventional electrode catalyst layer is operated for a long time under high temperature and low humidity conditions, the problems arise that cross leakage occurs as a result of the formation of pinholes in the polymer electrolyte membrane, and fluorine ions elute out. Consequently, sufficient durability cannot be achieved. This is thought to be due to chemical degradation caused by the perfluorocarbon sulfonic acid resin being attacked by hydroxyl radicals produced as a byproduct at either the anode catalyst or cathode catalyst (refer to A. B. LaConti, M. Hamdan and R. C McDonald, in “Handbook of Fuel Cells”, H. A. Gasteiger, A. Lamm, Editors, Vol. 3, p. 648, John Wiley & Sons, New York (2003)).
Proposed methods for improving the electrode catalyst layer include a method of incorporating fine particulate and/or fibrous silica in an anode electrode catalyst layer (see, e.g., Patent Document 1), a method of incorporating a fine particle of a crosslinked polyacrylate as a water-absorbent material in an electrode catalyst layer (see, e.g., Patent Document 2), and a method of comprising a metalloxane polymer in the electrode catalyst layer (e.g. refer to Patent Documents 3 and 4). However, even with these methods it has not been possible to suppress the elution of fluorine ions and durability has been insufficient.
Also proposed has been an electrode catalyst layer containing a polyfunctional basic compound (see, e.g., Patent Document 5). Although such compounds exhibit a slight improvement in durability, they still cannot be said to be sufficient. Furthermore, polyfunctional basic compounds such as hexamethylene diamine poison the electrode catalyst made of platinum or the like, whereby it has not been possible to obtain good power generation characteristics.
In addition, a cathode catalyst layer containing an anionic conductive polymer and a cationic conductive polymer has also been proposed (see, e.g., Patent Document 6). In a proposed production method for this, a cationic conductive polymer and anionic conductive polymer in solution are both mixed with a support catalyst, and the resultant mixture is sprayed onto a membrane and hot pressed (see paragraph 0025 of Patent Document 6). If the anionic conductive polymer is polybenzimidazole, to prepare this polymer in solution it is necessary to dissolve it in a high boiling point aprotic solvent such as dimethylacetamide. However, such a high boiling point aprotic solvent will remain in the electrode catalyst layer and poison the electrode catalyst made of platinum or the like, and thus it has not been possible to obtain good power generation characteristics.
Non Patent Document: M. S. Wilson and Gottesfeld, Journal of Applied Electrochemistry, 22, p. 1 to 7 (1992)
Patent Document 1: JP-A-6-111827
Patent Document 2: JP-A-7-326361
Patent Document 3: JP-A-2001-11219
Patent Document 4: JP-A-2001-325963
Patent Document 5: JP-A-2002-246041
Patent Document 6: JP-A-2004-512652